I have 4 fish, all young and none bigger than 2 inches. I have a yellow lab cichlid, a damonsoni cichlid, a skinny orange algae eater, and a small species of pleco.

I plan to eventually move to a bigger tank once these fish grow a little, but I'm really wanting to add some more fish. I'm wondering if adding fish is a bad idea or if there are some other fish that would be compatible.

Does anyone have some good guidelines to stick to in order to know when it's time for a bigger tank? Could I still add some fish, and if so what species will have the best chance of getting along with the ones I have?

I've had my tank going for just over 6 months and the fish I have get along great. I think the two cichlids are like best friends, which is a bit unexpected from my experience in the past with others, but those were South American, maybe African's get along better, not sure.

most cichlids grow fast, like now would be a good time for a bigger tank.
go for a tank that suits the breed you are getting and because most cichlids are territorial more so if they breed, i wouldnt over stock, probably better to understock to stop problems before they arise.
maybe worth getting a partner for each of you cichlids, i nearly brought damonsoni cichlids but my 190L wasnt going to be big enough, think a 55usgal is recommended for 1, but you may end up with 4 cichlids if you get each of your cichlids partners, or pick your favourite and sell the other.
though that is my opinion it wont be everyone elses, it is your choice abot what you would like to see for your tank, cichlids do have personality and they make a great species tank so there is no real need to add lots of variety.
if you can get them to breed they are great to watch as how they tend to their fry is amazing, they love their kids

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190L corner with a variety of fish i will not mention as its ever changing and i cant be bothered to keep editing a sig

You need to get those guys into a tank of at least 45 gallons. Yellow Lab and demasoni will get about 7 inches. Cichlids actually do better in overstocked tanks, it curbs some of the aggression. If you give them too much room they will claim territory and become aggressive. Obviously weekly water changes are necessary if you overstock